knife-edge

from WordNet (r) 3.0 (2006)

knife-edge
    n 1: a narrow boundary; "he lived on a knife-edge between genius
         and insanity"

from The Collaborative International Dictionary of English v.0.48

Knife-edge \Knife"-edge`\, n. (Mech.)
   A piece of steel sharpened to an acute edge or angle, and
   resting on a smooth surface, serving as the axis of motion of
   a pendulum, scale beam, or other piece required to oscillate
   with the least possible friction.
   [1913 Webster]

   {Knife-edge file}. See Illust. of {File}.
      [1913 Webster]